An electret is a static DC field carrier dielectric material which has a quasi-permanent charge with a surface potential field. Because of this, an electret could also be considered as an electrostatic carrying dielectric material similar to a permanent magnet. Whereas the parameter of a permanent magnet are very stable and cannot be affected by their surrounding environmental conditions, the surface charge potential of an electret decays over time. The surface charge potential is also affected by the surrounding environment, such as humidity, the density of nearby airborne ions, and electric fields, as well as by intrinsic parameters of the electret material, such as permittivity and resistivity.
Electrets can be created by heating up dielectric material at or near its melting temperature under a strong static electric field. The terms “electret material” and “dielectric material” are therefore used interchangeably herein. Most polymers may typically serve as a suitable dielectric material. When a dielectric material is at room temperature, the internal dipole elements are generally positioned randomly may move irregularly inside the dielectric material, and the dipole elements may be moved into temporary alignment by application of a static electric field. When the static electric field is turned off, the dipole elements return to their previous random positions and irregular movements. However, when the temperature of a dielectric material is increased close to or near the melting point of the material, the high temperature enables to the irregular movements of the dipole elements to increase. The increased movement of the dipole elements, when combined with a strong external static electric field, such as one from an extra high tension (EHT) power supply, induces greater alignment between the dipole elements, so that they are more closely aligned with the polarity of the applied static electric field. When a dielectric material at such an increased temperature has been subjected to a strong external static electric field and allowed to cool down to room temperature while remaining within the external electric field, the dielectric material may retain the induced alignment between the dipole elements. As the charged dielectric electret material cools down to room temperature (about 77° F.), the external static electric field serves to maintain the positioning and alignment of the dipole elements within the material, to the point that when the material returns to room temperature, the dipole elements may substantially retain the positioning and alignment induced by the strong external static electric field. With the dipole elements inside the dielectric material now in an induced alignment, the resulting electret has a semi-permanent electrostatic bias. This process of applying a strong external static electric field to a dielectric material while holding the dielectric material at or near its melting temperature is often referred to as a “corona static charge” method (hereinafter, simply the “corona method”).
At the microscopic level, the corona method induces the internal polarization of dipole elements to change from a random format into alignment along the electric field lines, theoretically forming ‘strings’ of sequentially aligned dipole elements. The strings stack up on top of each other, and they group together with other strings, to form a strong internal dipole electric field, which is in the opposite direction of the charging electric field, ε0. The corona method also results in a surface charge forming on the dielectric material. The surface field potential, εr, for the charged electret may be expressed as:εr=εsc−εdipole  (1)where εsc represents the field potential from charge deposited on the surface of the charged electret, and εdipole represents the field potential from the internal dipole elements. In standard practice, charged electrets are often wrapped by a piece of tin foil for a period of several days in order to remove charge deposited on the surface of the charged electret, thereby driving the εsc term toward zero. Following removal of the surface charge, the resultant surface field potential, ε′r, for the charged electret may be expressed as:ε′r=−εdipole  (2)
As indicated above, a charged electret has a surface field potential which is not stable and is affected by the surrounding environment and the inherent properties of the dielectric material. In other words, in order for an electret material to be truly useful, the surface field potential of the electret material needs to be restored. However, in general practice, restoring the surface field potential of a decayed electret involves applying a strong external static electric field, such as from an EHT power supply, in a high temperature environment.
When the surface field potential of an electret drops down to a low potential level, for example less than 1 kV, the surface field potential generally needs to be recharged, preferably back to the original surface filed potential, so that the electret may be useful once again. To accomplish this, the corona method may be used. However, while the known corona method may be quite feasible in an industrial setting, obtaining the necessary high temperature environment for the known corona method is highly impractical in other settings, such as for domestic users. Therefore, it is desirable to have a method for restoring the surface field potential of an electret material at or near room temperature.